At present, a lithium secondary battery is generally employed as an electric source for driving small electronic devices. The lithium secondary battery essentially comprises a positive electrode, a non-aqueous electrolytic solution, separator, and a negative electrode. A lithium secondary battery utilizing a positive electrode of lithium compound oxide such as LiCoO2 and a negative electrode of carbonaceous material or lithium metal is preferably used. As the electrolytic solution, a cyclic carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is preferably used.
Under the conditions, Japanese Patent Provisional Publication 7-37613 provides an electrolytic solution for lithium secondary batteries that employs an electrolytic solution comprising a combination of a common non-aqueous solvent (e.g., EC, PC) and a non-aqueous tertiary carboxylic ester solvent such as methyl trifluoroacetate (MTFA) or methyl pivalate (MPA), is stable in a voltage range higher than 4 V, and shows high electroconductivity in a temperature range of 0° C. or lower, low reactivity with lithium and long charge-discharge cycle life.
In the Japanese Patent Provisional Publication 7-37613, there are described a negative electrode of glassy carbon and a non-aqueous solvent of a mixture of PC and MTFA, a mixture of EC and MTFA, and a mixture of PC and MPA.
According to the study of the present inventors, however, it has been found that when generally employed carbonaceous material such as natural graphite or artificial graphite, especially highly crystallized carbonaceous material such as highly crystallized natural graphite or artificial graphite, is employed for the formation of a negative electrode in a lithium secondary battery, an electrolytic solution employing the above-mentioned non-aqueous mixture solvent is apt to decompose on the negative electrode to cause increase of irreversible capacity, and further sometimes causes exfoliation of carbonaceous materials. The increase of irreversible capacity and exfoliation of carbonaceous material are caused by decomposition of the solvent in the electrolytic solution at the time of charging and by electrochemical reduction of the solvent on the interface between the carbonaceous material and the electrolytic solution. Particularly, PC (propylene carbonate) having a low melting point and a high permittivity and showing a high electroconductivity even at a low temperature decomposes in the presence of a graphite negative electrode and therefore it cannot be used for a lithium secondary battery. Further, it has been found that EC (ethylene carbonate) also decomposes in portion in the course of repeated charge-discharge procedures so that the battery characteristics lower. It has been furthermore found that since methyl pivalate has a boiling point of 101° C., a non-aqueous solvent comprising 50 wt. % or more of methyl pivalate causes expansion of the battery and lowering of battery characteristics at elevated temperatures.
It is also found that the use of an electrolytic salt of LiClO4 (which is described in the Japanese Patent Provisional Publication 7-37613) in the electrolytic solution causes decomposition of the electrolytic solution to emit gas in the operation of battery at a high temperature and gives harmful effect to cycle property at 0° C. or higher.
Japanese Patent Provisional Publication 12-182670 describes a lithium battery which utilizes as the non-aqueous solution such a compound as ethyl acetate, methyl propionate or methyl butyrate and is employable at low temperatures. The non-aqueous battery described in the Japanese Patent Provisional Publication 12-182670 certainly shows good characteristics at low temperatures. However, a carboxylic acid ester having a structure in which a hydrogen atom is attached to a carbon atom placed adjacent to a carbonyl group, such as ethyl acetate, methyl propionate or methyl butyrate, is apt to produce a gas upon reaction with lithium metal (which is produced on the negative electrode by electrodeposition) and causes lowering of cycle property and lowering of storage endurance, in the case that it is employed in combination with a negative electrode of highly crystallized carbonaceous material such as natural graphite or artificial graphite in a lithium secondary battery. Further, in the case of employing a non-aqueous solvent containing 20 wt. % or more of methyl propionate (boiling temperature: 79° C.), expansion of the battery and lowering of the battery characteristics are observed.
Japanese Patent Provisional Publication 9-27328 describes that a non-aqueous electrolytic solution containing methyl decanoate, dodecyl acetate, or the like easily permeates into a separator, and that the non-aqueous battery utilizing such non-aqueous electrolytic solution shows large battery capacity and high battery voltage, and further shows little fluctuation of battery characteristics. The non-aqueous battery described in the Japanese Patent Provisional Publication 9-27328 certainly shows good permeation into an electrolytic solution into a separator, large battery capacity and high battery voltage. However, in the case that the disclosed electrolytic solution is used in combination with highly crystallized carbonaceous material such as natural graphite or artificial graphite in lithium secondary batteries, the solution may decompose on the negative electrode to cause increase of irreversible capacity and sometimes causes exfoliation of carbonaceous material. The increase of irreversible capacity and exfoliation of carbonaceous material are caused by decomposition of the solvent in the electrolytic solution in the charging process and originate from electrochemical reduction of the solvent on the interface between the carbonaceous material and the electrolytic solution. Particularly, a carboxylic acid ester having a structure in which a hydrogen atom is attached to a carbon atom placed adjacent to a carbonyl group, such as methyl decanoate or dodecyl acetate, is apt to decompose in portion in the course of the repeated charging-discharging procedures and causes lowering of cycle property in the case that the graphite negative electrode is employed.